I. Field of the Invention
This invention relates to a spring clutch for switch or step drives, such as for form feeds in office machines, having a wrap-around coil spring coupled for rotation with a drive hub, and having an unstressed diameter which is greater than the diameters of both the drive hub and a driven hub.
II. Description of the Prior Art
There are two basic constructions for clutches in form feeds or the like to provide positive connection between the spring and the driven and the drive hubs commonly sold. In one case the unstressed diameter of the spring is smaller than the diameter of the clutch hub. The free end of the spring is held stressed by a releasable catch, the spring suddenly winding up around the rotating drive hub when the catch is released and, as a result of the friction, is rotated with the hub. For declutching, the free end of the spring again engages the catch, so that the spring can be unwound to its stressed, open position again, by inertia or spring action. In the second case the unstressed diameter of the spring is greater than the clutch hub. To engage the clutch, the free end of the spring is pressed against the stationary hub, for example by means of a guide shoe, so that the spring draws together and the two hubs are coupled.
Although these clutch constructions may operate satisfactorily in many applications, there are, however, also cases in which the requirements with regard to the operating accuracy are particularly high and in which only small control forces are available for actuating the clutch. Such applications arise for example, in office machines in which record supports must be conveyed line by line and each line space has to be reproducible as accurately as possible. Accurate clutch actions are also required in other fields, for example in conveyor systems in the manufacturing industry and in sorting equipment.
The known spring clutches have the disadvantage that their switching position is largely determined by the time and the duration of the control force. Moreover, the duration of the controlling force is usually not the same as the duration for which the clutch is engaged, so that relatively great inaccuracies are introduced by this fact alone. In order to reach a pre-determined objective after declutching, a high motor force is therefore usually required so that the clutch still achieves its objective even when the drive is already interrupted. It has to be recognized that such restrictions have a direct influence on the operating accuracy and the inaccuracies introduced can possibly be additive.